This invention relates to extruders and injection machines of the type in which a screw rotatable within a barrel is employed to extrude or inject resinous material from the outlet end of the barrel.
A plasticating extruder receives polymer pellets or powder, heats and works the polymer sufficiently to convert it to a melted or plastic state and delivers the melted polymer under pressure through a restricted outlet or die. It is desirable that the extrudate be fully melted, homogeneously mixed and uniform in temperature and viscosity.
The basic extruder apparatus includes an elongated cylindrical barrel which may be heated at various locations along its length and a screw which extends longitudinally through the barrel. The screw has a core with a helical flight thereon and the flight cooperates with the cylindrical inner surface of the barrel to define a helical valley for passage of the resin to the extruder outlet opening or injection front chamber. Although the pitch of the flight can vary, it is common to utilize screws of constant pitch. The pitch is the forward distance traversed in one full revolution of the flight. The screw is rotated about its own axis to work the plastic material and force it toward the outlet end of the barrel.
An extruder screw ordinarily has a plurality of sections along its axial extent with each section being suited to the attainment of a particular function. Ordinarily, there is a feed section, a transition section and a metering section, in series. In some extruders, these sections are followed by a vent section plus a second transition section and a second metering section. The extruder screw feed section extends beneath and forwardly from a feed opening where polymer in pellet or powder form is introduced into the extruder to be carried forward along the inside of the barrel. The depth of the helical valley of the screw in the feed section is usually large enough to over-feed the solid polymer. The over-feeding action serves to compact and pressurize the polymer particles and form a solid bed of advancing material in the extruder.
The material is worked and heated so that melting of the polymer occurs as the material is moved along the screw. Solids conveying and compaction occur in the feed section. Most of the melting occurs in the transition section. Most of the melting occurs near the barrel surface at the interface between a thin melt film and the solid bed of polymer. This general pattern persists until a substantial portion of the polymer reaches the molten state. After some 40 to 80 percent of the polymer has been melted, which usually occurs in the transition section, solid bed breakup usually occurs, and at this time particles of solid polymer become dispersed in the polymer melt. From this point on, it often is advantageous to intimately mix the polymer melt with the unmelted material to accelerate melting and minimize local non-uniformities.
The melt is passed through the transition section wherein the root depth of the helical passageway is reduced to reflect the volume reduction due to melting of the feed. The reduction of depth in the transition section also compresses the solid. The transition section leads to a metering section, which has a shallow root depth helical passageway. Generally, the metering section begins where the resin is at least 90 percent melted.